June 15, 2011

It has been a long winter here in the Sierra Nevada. This "spring" was one of the coldest and wettest on record, and as a consequence the higher portions of the mountains are still blanketed in snow. But now that the melt has finally begun in earnest, my field season is fast approaching. Last week I hiked into the Golden Trout Wilderness to have a look at a particularly important population of the southern mountain yellow-legged frog (Rana muscosa). This population is the furthest south R. muscosa population left in the Sierra Nevada. Populations historically occurred as far south as Breckenridge Mountain in Kern County, and included large populations in the montane and subalpine meadows along the South Fork Kern River and Golden Trout Creek. As far as we know, those are all gone now, and this population that I visited is the last hold-out in this southern portion of the historic range.

The fact that this population is still extant is remarkable. Most of the meadows in the Golden Trout Wilderness have been badly degraded by intensive sheep and cattle grazing in the late 1800s and early 1900s that resulted in channel incision and lowered water tables. As a consequence, many of the ponds and other amphibian habitats disappeared. In addition, California golden trout (that were native to much of the South Fork Kern River watershed) were moved into the few naturally fishless lakes and streams that existed in the area, further reducing habitat for R. muscosa. And then probably sometime in the 1970s or 1980s, the amphibian chytrid fungus (Batrachochytrium dendrobatidis) swept across the region, further decimating R. muscosa populations.

Despite all of these changes, this population of frogs somehow hung on. But for how much longer into the future they will remain is an open question. This R. muscosa population is centered on a single small breeding habitat, an abandoned stream channel filled with sedges and fed by seepage from the adjacent stream. When the stream someday moves back into this channel (as it inevitably will), the only breeding site will be gone and so too will be the frogs. As such, it is imperative that frogs from this population be used to reestablish populations in suitable habitats elsewhere in the vicinity. Based on the low success rate of developing new mountain yellow-legged frog populations, this won't be an easy task. But without such an effort, the range of R. muscosa will continue to contract until this once abundant species is all but gone from its former haunts.